Blue clove polyps. If I could turn back the hands of time, I would have removed the frag that mine came in on and lit it on fire. I've had blue clove polyps for year and years and they aren't bad, per say, they just tend to cover everything. Everything. They don't sting much so when acros and other corals are growing out, there is always a tiny space between the two but as soon as something starts to fade, the clove's march in like WWII Nazis.
On a more positive note, your do appear to be a little different than mine. The If I were you, I'd pull the frag and scrape them off. Here's the important part - scrape them off outside the tank. For the first few years I didn't mind the occasional patch of blue cloves but at some point I decided to thin them back. The genius level think I am took a toothbrush to them and it was really, really effective. I was able to scrub off a ton of the little buggers. A few weeks later I had patches sprouting up everywhere. The only tank level treatment I've rad bout to rid one's self of this plague is fenbendazole. I have some on hand and I've been going back and forth because the treatment is also really effective at killing all kinds of other things. It's a toss up IF I'll ever actually treat for them because they really aren't that big of a deal.
Coating them with kalk paste or reef napalm really does a great job of killing them back without spreading them everywhere.